Quantum crypto edges closer

British boffins have made a breakthrough in quantum cryptography, an advanced code-making technology which is theoretically uncrackable, by developing a single photon-emitting diode.

The researchers from the University of Cambridge and Toshiba have discovered a way of incorporating semiconductor nano-technology into an LED so they can trigger the emission of single photons at regulated times.

This is important because the security of optical quantum cryptography relies on sending a single photon carrying digital information between two parties exchanging encoded information.

The laws of quantum physics dictate that an eavesdropper could not measure the properties of a single photon without the risk of altering those properties. This means a legitimate receiver of a message can test whether it has been intercepted or altered by a hacker during transmission.

In contrast to methods based on codes, the keys formed by quantum cryptography can, in principle, be completely uncrackable.

Until now, quantum cryptography has not been fail-safe, because conventional LEDs and lasers sometimes (unavoidably) produce two or more photons, which could allow a hacker to determine parts of the key without detection. Thus the single photon LED is crucial to ensuring the unconditional security of quantum cryptography.

"Single photons are rather like the magic bullets in molecular biology, in that the laws of quantum mechanics result in the certain failure of any attempt to intercept the information," said Professor Michael Pepper, managing director of Toshiba's Cambridge Research Laboratory.

Although the single photon LED might be produced relatively cheaply there are still barriers to be solved in the development of quantum cryptography systems. The single photon LED only works at low temperatures and attenuation effects on optical cables mean that the distances over which data can be exchanged by quantum cryptographic techniques are limited. ®

Boffin note

Here's Toshiba's explanation of how the technology works:

"At the heart of the Single Photon Emitting Diode lies a tiny volume of semiconductor, called a quantum dot. The quantum dot, which typically measures about 15nm in diameter and 5nm in height, is so small that it can capture at most only two electrons and two holes from the applied current pulse.

Recombination of a single electron and a single hole in the dot results in the emission of a single photon. We control the level of the applied current pulses so that the dot captures on average one electron and one hole, and thus produces one photon per pulse.

If the dot by chance captures two electron-hole pairs, it will emit two photons.

However, since the extra photon is emitted at a different wavelength, we can block it using a filter. Thus, in this mode of operation, we can ensure that only one photon is emitted per applied current pulse."

The advance will be reported in this week's edition of Science, the research journal.